The present invention relates to a braking force control system which improves stability of an automotive vehicle by applying braking force to a suitable wheel thereof when the vehicle is negotiating a tight cornering.
In recent years, various braking force control systems have been proposed for improving the stability of an automotive vehicle by applying braking force to a proper wheel thereof when the vehicle is cornering on the basis of the force applied to the vehicle during the cornering.
For example, Japanese Patent Laid-Open No. 2-70561 discloses a braking force control system which controls the braking force on the basis of rotational motion around the vertical axis passing through the center of gravity of an automotive vehicle, i.e., on the basis of a yaw rate which is an angular velocity of yawing. In this technique, a target yaw rate is compared with an actual yaw rate, and then, it is determined as to whether the vehicle running condition is an under-steering tendency or an over-steering tendency with respect to the target yaw rate. Braking force is applied to the inside wheels when it is the under-steering tendency, and braking force is applied to the outside wheels when it is the over-steering tendency, so that the running stability of the vehicle is improved.
However, in the aforementioned prior art, for example, if braking force is applied to the front-inside wheel due to the under-steering tendency during the cornering of a vehicle, there is a problem in that the under-steering tendency is intensified when the front-inside wheel tends to be locked due to the decrease of frictional resistance against a road surface and so forth. To the contrary, if braking force is applied to the rear-outside wheel due to the over-steering tendency, there is a problem in that the over-steering tendency is intensified when the rear-outside wheel tends to be locked.
As another example for selecting a wheel to be braked, Japanese Patent Laid-Open No. 4-372447 discloses that a wheel to be braked is selected on the basis of steering conditions. However, this can not also eliminate the aforementioned problems.
Moreover, it is required to strictly select a wheel to which braking force is to be applied, from four wheels in accordance with conditions. In a case where a rear wheel is selected as the braked wheel, according to the values calculated on the basis of the vehicle running condition and the difference in yaw rate, although the braking force applied to the rear wheel can assure a stable control on a high .mu. road surface, it may reduce the stability of the vehicle on a low .mu. road surface since the side slip of the rear wheel is caused by the braking force. In addition, if the braking force applied to the rear wheel is calculated by using the same gain as that of the braking force applied to the front wheel, the moment of turning-round increases so as to give an unnatural feeling to the driver. Furthermore, Japanese Patent Application Laid-Open No. 5-270382 discloses that the braking force is regarded as important on a high .mu. road surface to ensure the braking force is applied to the rear wheel, and that the braking force to be applied to the rear wheel is decreased on a low .mu. road surface in accordance with the increase of the difference between a target yaw rate and an actual yaw rate in order to prevent the loss of the stability due to the side slip of the rear wheel by the braking force of the rear wheel. This relates to the distribution of braking force when a vehicle turns in a system which controls the distribution of the braking force to the right and left wheels and the braking forces applied to the two rear wheels (simultaneously).
The aforementioned braking force control for improving the vehicle running stability is carried out on the basis of the difference (the difference in yaw rate) so that the actual yaw rate is coincident with the target yaw rate, and a non-control region is set so that the control is carried out only at a larger difference in yaw rate than a predetermined value in order to prevent the unnecessary control at a small value of the difference in yaw rate. That is, when the control is designed to be carried out at a small value of the difference in yaw rate, the control is carried out even if the driver can easily cope with the cornering, so that an unnatural feeling is given to the driver and the abrasion of brake parts is increased. Therefore, in this condition, the control is designed not to be carried out.
On the other hand, if the non-control region is set in a larger range than the necessary range, the controlled conditions are decreased, so that the effects of the control can not be fully exhibited.
Therefore, various techniques for suitably setting the non-control region have been proposed. For example, Japanese Patent Laid-Open No. 6-239216 discloses that the non-control region is so set as to vary in accordance with vehicle speed since the suitable non-control region varies in accordance with vehicle speed.
In general, as shown in FIG. 1, when an automotive vehicle turns (FIG. 1 shows an example where an automotive vehicle turns to the right, wherein it is assumed that the sign in the direction turning to the right is plus (+), and the non-control region is expressed by the slanting lines between -.epsilon..ltoreq..DELTA..gamma..ltoreq..epsilon..), an actual yaw rate .gamma. is first in an under-steering tendency with respect to a target yaw rate .gamma.' (t1&lt;t&lt;t4), and then, it is changed to an over-steering tendency with respect to the target yaw rate .gamma.' (t4&lt;t&lt;t7) to converge at the target yaw rate .gamma.' (FIG. 1(a)). The absolute value of the difference in yaw rate .DELTA..gamma. (=.gamma.-.gamma.') is decreased to be a value within the non-control region when the actual yaw rate .gamma. changes from the under-steering tendency to the over-steering tendency. Therefore, the control is not carried out during this change, and the control is started a while after the actual yaw rate .gamma. is changed to the over-steering tendency (FIGS. 1(b) and 1(c)). As a result, since the control is discontinuous, there is a problem in that an uncomfortable feeling is given to the driver if the control is started a while after the actual yaw rate .gamma. changes to the over-steering tendency. Since this problem is caused even if the vehicle speed is constant, it can not be solved by the aforementioned technique for setting the non-control region in accordance with the vehicle speed.
In particular, when the control is carried out by applying braking force to the rear wheel, the side slip of the rear wheel may be caused. Therefore, it is desired to restrain the control, which is carried out by applying braking force to the rear wheel, as much as possible.